The major point to remember is that all letters of a given typeface and a given size fit into rectangles that are the same height.
I normally spend a lot of time on this in my classes at school because it is an important concept to understand (and many designers never really get a handle on it). Often paragraphs or lines of type look very different in size, but in fact they are the sa me point size. This is primarily due to variations in x-height and built-in leading. Any letter that goes exceptionally high or low changes the size of the entire typeface.
For example, examine the graphic above as carefully as possible. As you can see, there are huge differences in x-height between Ardone and Bernhard Modern. Bernhard Modern has a lot of built-in leading also. Even though Ardone and Futura have very similar x-heights, you can see that Futura has no extra leading built into the font, whereas Ardone has some. The result is that 10-point Futura will look as large as 11-point Ardone (and maybe 13- or 14-point Bernhard Modern). These things must be taken into account when you pick your fonts for your projects.
This will be confusing for a little while. However, if you remember these few ideas, you’ll be able to understand much better how type works in your software. The options will begin to make sense. They are something you need to control.
Leading was traditionally measured from baseline to baseline. In other words, leading was the space needed to leave room for the next line, measured from the baseline of the original line to the baseline of the following line. To use typewriter imagery: when you hit the carriage return, the roller advanced the distance necessary to allow the next row of type to be typed without overlap.
Type speak: Point size and leading are usually written as a fraction. The point size becomes the numerator and the leading the denominator. Normal body copy is written 10/12. This would be spoken as: ten on twelve — meaning ten-point type with twelve points of leading.
As mentioned, many fonts already have built-in leading. You need to be aware of things like this when you pick a font. A font like Futura has almost no built-in spacing and therefore needs to be set with extra leading for readability (as if you could read Futura anyway).
A font like Bernhard Modern has so much built-in leading you might be tempted to use negative leading as in 14/13. If you are using a font that is all capital letters, you may want to set the type as 24/18 or so. If you do not, there will be too much space between the lines with a third of the point size blank because there are no descenders.The leading slug
I should mention that the meaning of the word slug has changed. It used to be a blank piece of lead the size of the type. But it has a different meaning digitally.
The digital slug: While you are getting used to your software, it is helpful to highlight your type and examine the result. The height and vertical location of the highlight box containing the reversed-out type shows the leading, and is called the slug. This is the easiest way to see how your font fits the leading. Built-in spacing can also be seen in this manner.
The main thing about leading is that it greatly affects readability. Normally, the longer the line length, the more leading is required
(that is 10 point type should be set around 4" wide, ± a half inch or so.
Another old letterpress term you will hear is set solid. Type with no extra leading as in 12/12 is referred to as being set solid. That is, in letterpress, the character slugs were set on top of each other with no extra strips of lead.
The first aspect of sizing type is the ruler. Those who are terrified of metric can relax — type sizing is much more irrational. What happened was this. Until the mid-1800s, all type foundries had their own sizes.
These sizes were given common names, but there was no universal standard. Some of the names will still be familiar to you. Agate, for example, is a small size used for classified ads (now 5.5-point is used). Other common type sizes were diamond, minion, brenier, long primer, great primer, and canon.
Around 1875, a man named Nelson Hawkes decided to rectify the problem. He came up with a measurement system based on one of the more common sizes, the pica. In the early 1700s, King Louis XV had established the point as the standard type measurement for printers in France. Pica type happened to be 12 points high.
However, by the time Nelson made his plans, there were two points: the European point, which was .0148 inches; and the American point, which was .0138 inches. This made 72 European points be a little more than an inch, and 72 American points were a little less than an inch.
Using the American point, Hawkes decided that 12 points should be called a pica and built an entire type sizing system upon picas. All type came to use this measurement system.
Points were an excellent sizing tool. At approximately 72 points per inch, the smaller sizes of body copy could be clearly differentiated. Type that is one point larger or smaller is almost the smallest increment of size that can be distinguished with the naked eye. We can see, with the naked eye, the differences between 9 and 10-point, 12 and 13-point. We cannot see the difference between 11 and 11.5-point type.
Within a few years, picas and points became the American standard. Today all type is sized in points. The computer has helped. One of the developments in type sizing that eased things a bit for Americans was brought about by Apple’s Macintosh. When Apple came out with the Mac and its GUI, they set the screen resolution at 72 pixels per inch.
In the years since 1984, the 72-point-per-inch standard has become universal on desktop computers. This is true even though high-resolution monitors make this measurement meaningless. The nice thing, for us Americans, is that this is exactly 72 points per inch whereas, under the pica system 72 points came to .9936 inch.
This insignificant difference meant that 66 picas was not 11 inches, but 10.93 inches (a little more than 10-7/8 inches). This caused amazing havoc for traditional pasteup artists and designers, whose art never fit the way they designed it. For years, many artists drew all their boards in inches, set all their type in picas, and tried to force things to fit (they often didn’t).
At this time the pica is disappearing. In fact, we can safely say it is gone in most cases (except for newspapers). But points probably will never go away. It is too useful. One of the first things you do when you install a program is decide which measurement system you are really going to use: inches, pixels, or millimeters. As far as I know, the only industry still using picas is newspapers, and they only use it for column widths.